1. Field of the Invention
This invention relates to laser systems and associated fabrication methods for directing a laser beam away from a substrate, and more particularly to the use of an external turning mirror to redirect an in-line beam from a monolithically fabricated laser away from its substrate.
2. Description of the Related Art
Ultra-high speed interconnect links between integrated circuit (IC) chips and data busses are needed for 3-dimensional optoelectronic systems. Currently available electronic systems incorporate optical isolation and optical data paths between larger subsystems, but the optical elements are discrete. A more compact, less expensive and more reliable system would result if the optical elements could be monolithically integrated on the same chip substrates as the electronic circuitry as surface emitting (optical output directed away from the substrate surface) elements. Applications for such 3-D interconnects include computers and processors, optical displays, optical signal processing and computing, intersatellite communication, the pumping of solid state crystals, and visual displays.
Three approaches have been developed to achieve out-of-plane laser emissions:
(1) Vertical cavity lasers, in which the laser beam is initially emitted vertically upward and away from the substrate upon which the laser is formed. This type of laser is described in Tell et al., "High-power cw vertical-cavity top surface-emitting GaAs Quantum Well Lasers", Applied Physics Letters, Vol. 57, No. 18, 29 Oct. 1990, pages 1855-1857. They have very short optical cavities, on the order of an optical wavelength, and require high reflecting mirrors, typically using epitaxially grown Bragg reflectors for the mirrors. However, such lasers exhibit poor efficiency and high electrical resistance. Also, it is difficult to produce high reflectivity semiconductor quarter wavelength Bragg mirrors, due to the low index of refraction modulation in such mirrors.
(2) A periodic grating on the laser's upper cladding layer to couple light vertically out of the laser plane. See, for example, Itaya et al., "New 1.5 Micron Wavelength. GaInAsP/InP Distributed Feedback Laser", Electronics Letters, Vol. 18, No. 23, 1982, pages 1006-1007, and Ng et al., "Highly collimated broadside emission from room temperature GaAs distributed Bragg reflector lasers", Applied Physics Letters, Vol. 31, No. 9, 1 Nov. 1977, pages 613-615. Unfortunately, lasers of this type suffer from having extremely elliptical output beams and they require long sections of the grating, which increases the device size.
(3) In-plane surface emitting lasers, in which a laser beam is initially generated generally parallel to the substrate, and then deflected by a turning mirror so that the beam travels away from the substrate. The turning mirror can either be part of the laser cavity (a "folded-cavity" laser), or external to the laser.
An early folded-cavity laser contained a 45.degree. surface that was etched into the topside of the wafer to deflect light into the substrate; SpringThorpe, "A novel double-heterostructure p-n-junction laser", Applied Physics Letters, Vol. 31, No. 8, 15 Oct. 1977, pages 524-525. A disadvantage of this type of laser is that it must be made on a transparent substrate material, or have a deep via etched into the substrate. In addition, the device must be mounted top-side down, and thus is not readily integrated with electronic circuits, whose connections are typically formed by wire bonding to the top surface. Folded-cavity lasers with an emission from the top surface have also been demonstrated. The 45.degree. mirror of such lasers is made by dry-etching a slot into the top surface, which involves a more complicated procedure. See Goodhue et al., "Monolithic Two-Dimensional GaAs/AlGaAs Laser Arrays Fabricated by Chlorine Ion-Beam-Assisted Micromachine", Journal of Electronic Materials, Vol. 19, No. 5, 1990, pages 463-469.
Surface-emitted beams with good far-field patterns have been produced with folded-cavity lasers, since it is possible to form an integrated lens on the emitting surface of the device to collimate or focus the output beam. However, the design and fabrication of this lens is very complicated, since it must perform the dual functions of cavity reflection and beam shaping; see Liau et al., "GaInAsP/InP buried heterostructure surface-emitting diode laser with monolithic integrating bifocal microlens" Applied Physics Letters, Vol. 56, No. 13, 26 March 1990, pages 1219-1221.
Most surface emitting lasers incorporate external-cavity turning mirrors that are spaced from one or both ends of the laser cavity. Such a device is shown in Liau et al., "Surface Emitting GaInAsP/InP Laser with Low Threshold Current and High Efficiency", Applied Physics Letters, Vol. 46, No. 2, 15 Jan. 1985, pages 115-117. These lasers are fabricated by growing a conventional (parallel to the substrate) in-line laser structure, and then forming the vertical cavity mirrors and a 45.degree. tilted deflecting mirror that is opposite one of the cavity mirrors. A common method is to form both the cavity mirror and the turning mirror at one end of the cavity by ion-beam milling, with the wafer surface inclined away from normal to the incident beam direction (see Goodhue et al., cited above). The turning mirror can be provided with a curved surface to collimate or focus the beam which it deflects. A second milling step may be used to form the mirrors at the opposite end of the cavity, if desired. Since the laser cavity is similar to that of edge-emitting lasers, the device's performance is likewise as good as that of edge emitting lasers in most respects. However, these lasers suffer from a distorted far-field pattern and reduced output efficiency.
FIG. 1 shows a conventional horizontal cavity surface emitting laser system. A laser 2 extends upward from a semiconductor substrate 4, with an active lasing layer 6 sandwiched between upper and lower semiconductor cladding layers 8 and 10; the body of the substrate can itself serve as the lower cladding layer. The laser's rear surface 12 is coated with a fully reflective mirror (not shown), while an angled trench 14 is formed immediately in front of the laser to permit the deposition of a partially reflective mirror (not shown) over the front end 16 of the laser. The trench wall 18 opposite the laser is formed at an angle, typically 45.degree., that causes at least part of the emitted laser beam 20 to be deflected generally perpendicular to the substrate.
The laser's active (light emitting) layer 6 is typically located between 1 and 2 microns below the device's upper surface. Since the vertical height of the active layer is quite small, on the order of 0.1 micron, the out-put beam 20 (indicated by dashed lines) has an appreciable vertical divergence or "fanning". The beam will in general diverge vertically beyond the mirror surface 18, with as much as 20%-40% of the beam (indicated by shaded region 22) passing above the mirror and not being deflected along with the rest of the beam. This optical loss reduces the laser's efficiency, and distorts the resultant beam pattern. Interference effects from the light hitting the edges of the turning mirror also produce unwanted ripples or side lobes in the far-field pattern.